Referring to FIG. 1, heterodyne receivers 100 combines, by "mixing," (216) a received high-frequency wave, such as a radio frequency wave (210), with a locally generated wave (218) in a nonlinear device (216) to produce sum and difference frequencies at the output of the mixer (216). Mixers for such a receiving circuit can also be used for frequency conversion in transmitters. Furthermore, the modulation process employed in a transmitter is applicable to the demodulation process of a demodulator circuit.
Nonlinearity is required in any mixer, for the production of frequencies not present in the input, but any nonlinear device can serve as a mixer. Thus, diodes, transistors, saturable reactors, or any other type of nonlinear devices can be used as a mixer.
Design choices depend upon considerations of mixer performance parameters, such as gain (or loss), noise figure, stability, dynamic range and possible generation of undesired frequency components that produce intermodulation and cross-modulation distortion. Conversion Gain (Loss) is the ratio of the output (IF) signal power to the (RF) input signal power. Contributing to the sensitivity of the receiver, noise figure is defined to be the signal-to-noise ratio SNR at the input (RF) port divided by the SNR at the output (IF) port. Isolation represents the amount of "leakage" or "feedthrough" between the mixer ports. Conversion Compression relates to the RF input power level above which the curve of IF output power versus RF input power deviates from linearity. Above this good linearity level, additional increases in RF input level do not result in proportional increases in output level. Two-Tone, Third-Order Intermodulation Distortion is the amount of third order or harmonic distortion caused by the presence of an unwanted received signal at the RF port. The higher the conversion compressions the greater will be the suppression of this intermodulation product.
One mixer used in heterodyne receivers, is the passive balanced mixer using a balanced bridge diode configuration, as seen in FIG. 2. Because such a balanced diode mixer produces both sums and differences of the two input frequencies, it can be used as amplitude modulators and demodulators as well as mixers. Hence the terms "balanced modulator" and "balanced mixer" are synonymous. In mixers, the input frequencies will be of the radio frequency signal f.sub.RF and of the local oscillator signal f.sub.LO, resulting in the output frequency of the intermediate frequency signal f.sub.IF. Similarly, in modulators, the input frequencies will be of the carrier signal fc and of the modulating signal fm, and the desired output frequency will be at fc.+-.fm.
The passive balanced mixer of FIG. 2 is more specifically called a double-balanced mixer because it uses at least two nonlinear devices, such as diodes 51-54, with both the RF and LO inputs applied to separate ports 221-223 in a push-pull fashion so that neither signal appears at the other two ports. In other words, the LO signal (222) does not appear at, or is isolated from, the RF 221 or IF 223 ports, and so forth.
These four diodes 51-54 require a well-balanced input and output baluns 56 and 58 and accurate matching of the diode characteristics to provide a balanced output. Two wire-wound ferrite transformers 56 and 58 are typically used as the balun in ultrahigh frequencies (UHF). The trade-off for the high isolation provided is that this type of transformers is physically large as well as expensive. The degree of isolation between the three ports is achieved by how well these transformers are exactly center-tapped. It is assumed that the local oscillator voltage is large enough to control the on-off cycle of the diodes; that is, the currents due to v.sub.RF are small compared with those due to v.sub.LO such that the diodes act as switches.
The advantages of such a passive balanced mixer are that it has good linearity, port isolation, and can suppress even order spurious signals. However, this type of mixers has a high conversion loss of 6.5 dB at UHF frequencies. This high conversion loss results in a receiver that is not sensitive enough to meet the requirements of a low noise figure, low current specification.
Referring back to the receiver block diagram of FIG. 1, a bandpass filter 214, such as a microstrip split-ring resonator, is placed in front of the mixer 216 (and coupled by capacitors 215 and 217) to selectively attenuate the image spurious signal at f.sub.im =f.sub.LO +f.sub.RF and to remove all components except the desired one at f.sub.IF =f.sub.LO +f.sub.RF. As is known, a microstrip is a microwave transmission component in which a single conductor is supported above a ground plane while a stripline has two microstrips placed conductor-to-conductor with two ground planes on the exposed surfaces. Microstrip, strip-line ring resonators, and any other transmission line components are used in bandpass filter applications to overcome the influence that parasitic components generated at short circuited points in resonators have on circuit losses and resonance frequencies.
This microstrip split-ring bandpass filter typically has a loss of 2.5 db. Added to the 6.5 dB conversion loss of the balanced bridge diode mixer of FIG. 1, the combined loss of the two stages is 9.0 dB. This 9.0 dB loss is usually too high to allow the receiver to be sensitive enough without inserting an IF amplifier, adding in other components, or otherwise modifying the mixer, in a way, that may increase the mixer's intermodulation distortion. Thus, it would be advantageous to provide the functions of the passive balanced diode mixer and of the split-ring resonator filter but with less loss and sufficient attenuation at the image frequency.